Disallow db.delete() with arguments

db.delete() will delete entire database. If a user has a bad day and
tries something like db.delete(myObject); expecting it to delete a
certain object, the result must not be that entire DB is deleted.
Nested transactions derive transaction.source

Nested transaction will by default derive the source property from its
parent transaction. The source property can be used to mark the changes
a originated from a certiain source. It can be used when subscribing to
the db.on('changes') event that Dexie.Observable adds to Dexie. When
changes arrive the subscriber may inspect change.source to see if the
change was initiated by itself and the ignore that change. It is also
used by Dexie.Syncable to mark certain changes as originating from a
remote node so that the same changes are never sent back to that node.

This commit makes a nested transaction derive the source property from
its parent transaction by default. This makes it easier for application
code to know that the source property needs only to be set on parent
transaction and all nested function will derive it no matter whether
they enclose their operations within a nested transaction or not.

Also, lowering the NODE_TIMEOUT from 30 to 20 seconds.
Disallow empty "url" to db.syncable.connect()

Also: Let database be ready immediate if the sync operation is performed
by another node. Will eventually in a future checkin also let it be
ready immediate also when own node makes the sync. It's nice to have
access to DB right away and not having to wait for sync to complete.
After all, that's one of the benefits with a local syncronized copy of
the database.

Also, bugfix in the use of PersistedContext.save() from
finallyCommitAllChanges() where the _syncNodes table wasnt included in
transaction. Made the save() method independant on ongoing transaction
but resuse it if it is compatible. Still dont want to include that table
in finallyCommitAllChanges() because it's not vital for transaction
correctness and it would block pollers and cleanup code.
Deadlock bugfix in unit-test for transaction